286 



Scientific Proceedings, Royal Dublin Society. 



quickest flow of air (7,800 ccs. per miii.) tlie value 



n 



slowest (812 ees. per min.), ~ = B'l. 



(X 



1-06 ; for the 



MoMliiies met with in these Experiments. 



The mobility of the charged condensation nuclei varied with the rate of 

 flow of the air through the apparatus. The slower the rate of flow the 

 larger were the ions. The mobilities obtained varied from -So to -00024 cms. 

 per sec. under a field of 1 volt per cm. We obtained ions of mobilities -oo, 

 •18, -10, -02, -008, -0044, -0034, -0025, and -00024. The numbers were deduced 

 from the saturation voltages in current-volta,ge curves, and do not claim a 

 high degree of accuracy. The positive and negative ions under similar 

 conditions have the same mobilities. 



As an example of the manner in which the mobility varied with the rate 

 of flow of the air, the following numbers will be quoted. During the 

 experiment the intensity of the light, &c., remained constant, only the rate 

 of flow of the air varying : — 



Effeel of Varying the Intensity of the Light. 



C. T. E. Wilson observed that the nuclei produced by ultra-violet light 

 increased in size as the intensity of the light increased. Several experiments 

 were performed by us showing that by diminishing the distance of the 

 spark from the quartz tubes, and thus increasing the light intensity, 

 the nuclei grew in size and became more numerous. 



In these experiments, keeping the current of air constant, and a voltage 

 on the ionization ttibe sufficient to " saturate " the largest ions produced, the 

 distance of the spark was varied. One of the quartz tubes employed gave 

 an appreciable effect up to 15 mms. distance ; with the second the effect 

 stopped at 5 mm. Thus in oue experiment, calling the current due to the 

 uranium alone, Cj = 100, tlie following values were obtained for C™ with 



